Alchemical Free Energy and Hamiltonian Replica Exchange Molecular Dynamics to Compute Hydrofluorocarbon Isotherms in Imidazolium-Based Ionic Liquids

Ionicliquids (ILs) have shown promise for applications that leveragedifferential gas solubility in an IL solvent, e.g., gas separations.Although most available literature provides Henry's law constants,the ability to efficiently estimate full isotherms is important forengineering design calculations. Molecular simulation can be usedas a tool to predict full isotherms of gas in ILs. However, particleinsertions or deletions in a charge-dense IL medium and the sluggishconformational dynamics of ILs present two sampling challenges forthese systems. We therefore devised a method that uses Hamiltonianreplica exchange (HREX) molecular dynamics (MD) combined with alchemicalfree energy calculations to compute full solubility isotherms of twodifferent hydrofluorocarbons (HFCs) in imidazolium-based IL binarymixtures. This workflow is significantly faster than the Gibbs ensembleMonte Carlo (GEMC) simulations which fail to deal with the slow conformationalrelaxation caused by the sluggish dynamics of ILs. Multiple free energyestimators, including thermodynamic integration, free energy perturbation,and multistate Bennett acceptance ratio method, provided consistentresults. Overall, the simulated Henry's law constant, isothermcurvature, and solubility trends match experimental results reasonablywell. We close by calculating the full solubility isotherms of twoHFCs in IL mixtures that have not been reported in the literature,demonstrating the potential of this method to be used for solubilityprediction and setting the stage for future computational screeningstudies that search for the "best" IL to separate azeotropicHFC mixtures.